Nuclear medicine imaging techniques, which are represented by positron emission tomography (hereinafter “PET”), digital scintillator detector (hereinafter “DS”) and single photon emission computed tomography (hereinafter“SPECT”), for example, are effective in the diagnosis of diseases, including heart disease and cancer. These techniques involve administration of labeled tracers with a specific radioisotope (hereinafter “radiopharmaceutical”), followed by detecting γ-rays emitted from the tracers. Nuclear medicine imaging techniques have been widely used in medicine because of their high specificity and sensitivity to diseases. Such techniques also generally provide a high degree of information about diseases, compared to other diagnostic techniques.
In recent years, a series of radioactive halogen-labeled compounds, including [18F]1-amino-3-fluorocyclobutanecarboxylic acid (hereinafter “[18F]FACBC”), have been discovered and developed as radiopharmaceuticals, and their clinical application is under investigation (e.g., Japanese Patent Laid-open No. 2000-500442; Jonathan McConathy et al., “Improved synthesis of anti-[18F]FACBC: improved preparation of labeling precursor and automated radiosynthesis”, Applied Radiation and Isotopes, (Netherlands). 2003, 58, p. 657-666; and Timothy M. Shoup et al., “Synthesis and Evaluation of [18F]1-Amino-3-fluorocyclobutane-1-carboxylic Acid to Image Brain Tumors.”, The Journal of Nuclear Medicine, 1999, 40, p. 331-338). [18F]FACBC is considered to be effective as a diagnostic tracer for highly proliferative tumors because it has a property of being taken up specifically by an amino acid transporter.
The process for preparing [18F]FACBC comprises: providing 1-(N-(t-butoxy-carbonyl)amino)-3-[((trifluoromethyl)-sulfonyl)oxy]-cyclobutane-1-carboxylic acid methyl ester as a labeling precursor; substituting the triflate group at position 3 of the precursor with a radioactive fluorine; and carrying out deprotection by subjecting the resulting compound to an acidic condition (e.g., Japanese Patent Laid-open No. 2000-500442; Jonathan McConathy et al., “Improved synthesis of anti-[18F]FACBC: improved preparation of labeling precursor and automated radiosynthesis”, Applied Radiation and Isotopes, (Netherlands). 2003, 58, p. 657-666; and Timothy M. Shoup et al., “Synthesis and Evaluation of [18F]1-Amino-3-fluorocyclobutane-1-carboxylic Acid to Image Brain Tumors.”, The Journal of Nuclear Medicine, 1999, 40, p. 331-338).
However, the cost of preparing a compound labeled with [18F] is high, and the compound labeled with [18F] is limited to specific compounds (e.g., the compound cannot be labeled until some groups in the compound are substituted by a fluorine or a carbon).